1. Field of the Invention
The present invention relates to a decision feedback equaliser (DFEs) which comprises a feedforward part whose output is connected to an input of a decision stage and a feedback part connected between the output and input of the decision stage so that the inputs to the feedback part comprise previously detected symbols. The present invention also relates to a method of operating the DFE and to a receiver including the DFE.
2. Description of the Related Art
Equalisers are known per se and are used in communications receivers for enabling decisions to be made as to the nature of the data symbol which has been distorted by transmission through a dispersive channel which may comprise a cable or a radio link. Types of equalisers other than a DFE are linear equalisers (LEs) which comprise a feedforward filter whose output is connected to a threshold detector (or decision stage) and Viterbi (or Maximum Likelihood Sequence Estimation (MLSE)) equalisers in which a received signal is compared with signatures stored in a memory bank and the best match is obtained.
Viterbi equalisers are regarded as having a better performance to DFEs but this is at the expense of the Viterbi equaliser being of more complex construction and consuming a relatively large current compared to a DFE.
D. D. Falconer, A. Sheik, E. Eleftheriou and M. Tobis in an article "Comparison of DFE and MSLE Receiver Performance on H. F. Channels" Globecom 1983, Conf. Rec. Nov. 1983 pp 1.3.1. reported obtaining some real channel measurements and calculated the theoretical bit error rates for a DFE and a MSLE equaliser and obtained the conclusion that on average the theoretical optimums for both are equal to within 1 dB. However no implementation of a DFE which approaches the theoretical optimum is known to have been made for radio channels.
Various studies have been made of decision feedback equalisers which were originally devised for telephone channels and are used to remove unwanted sections of the channel filter impulse response in order to cancel or reduce the effects of intersymbol interference. K. H. Meuller and M. Muller in an article "Timing Recovery in Digital Synchronous Data Receivers" IEEE Transactions on Communications Vol. Com.-24, No. 5, May 1976 pages 516 to 531 reported that in their opinion the energy in received signal samples can be maximised with respect to noise by adjusting a receiver clock until the clocking point is aligned with the peak of the impulse response of the channel. Consequently it has been customary when operating DFE's on telephone channels to synchronise the reference tap of the DFE to the peak of the impulse response.
In the present specification the expressions reference tap and reference tap position are to be understood as meaning a synchronisation marker between the transmitter and the receiver. The reference tap position serves to compensate for the time delays occurring between the instant of transmission of a currently detected symbol and the instant a decision is made to estimate the symbols.
H. Sari in an article entitled "Baseband Equaliser Performance in the Presence of Selective Fading" published by the IEEE Global Telecommunications Conference Proceedings 1983 pages 111 to 117 discusses LEs and DFEs and mentions that tap-gain optimisation in baseband equalisers is usually carried out by fixing the position of the reference tap (RTP) and then minimising the output mean square error (MSE) for that fixed RTP. Sari shows that by making the position of the reference tap adaptive, a considerable performance improvement can be achieved in both equaliser structures and especially in DFEs. In order to achieve RTP adaption Sari proposes the use of a second (or slave) equaliser for estimating the optimum RTP and transferring it to the main equaliser. At the commencement of operation the main equaliser will start with the centre tap reference, because it is is considered to be the optimum position in the absence of fading. The slave equaliser will periodically try the N reference tap positions and for each RTP the estimated output MSE will be compared to that obtained with the main equaliser and every time a smaller MSE is obtained with the slave equaliser, the position of the reference tap of the latter as well as its tap-gain values will be transferred to the main equaliser.
Sari's proposal for using a slave equaliser for determining the best RTP to be used by the main DFE is to optimise the operation of the DFE with respect to different degrees of fading. However Sari does not consider the problem of choosing a RTP which will give an acceptable performance of the DFE for differnt channel impulse responses which will occur with mobile radio equipment. Also, particularly in the case of portable equipment, it is desirable to minimise the current consumption.